Recently, an increasing number of vehicles are provided with a camera for taking images of lateral or rear areas of the vehicle so that a driver can visually recognize the images displayed on a screen provided inside the vehicle. Further, a system or a device that performs an image processing on the image taken by the camera for assisting a driving maneuver, such as parking, is developed. In particular, the camera requires a highly precise calibration, including adjustment of an optical axis. This is because the image taken by the camera serves as a base to generate information necessary for positioning of the vehicle. For example, JP2008-131250A (Reference 1) discloses a technology of calibration of an on-vehicle camera. According to Reference 1, the on-board camera takes an image of a marker (a calibration index) that is positioned at two points within a vision range of the on-board camera and has a black and white checkered pattern. The image processing is performed on the image taken by the on-board camera to detect a center point (a calibration point) of the marker.
The on-vehicle camera is calibrated when mounted on a vehicle in a vehicle assembly plant or the like so as to absorb an installation error. The on-vehicle camera is also calibrated in a repair shop or the like to adjust misalignment caused by accumulated vibration or impact due to travel of the vehicle. Commonly, the calibration of the camera is performed in a state where the vehicle is parked in a predetermined position. To park the vehicle in a correct position, the vehicle may be driven forward or backward until the wheels of the vehicle are stopped by a wheel stopper or a groove for accommodating the wheels.
In order to accurately calibrate the camera, it is inevitable to accurately park the vehicle in the predetermined position. In the vehicle assembly plant or the repair shop, a front end of the vehicle is detected by a phototube to have the vehicle parked with a high accuracy. In the meantime, the camera may be mounted on various vehicle models. Since an overall vehicle length varies depending on the vehicle models, a distance between the camera and the marker also varies when the vehicle is parked in a manner that the front end of the vehicle is aligned with a base line provided on the floor. When two vehicles having different overall vehicle lengths are parked in the manner that the front ends thereof are aligned with the base line, a distance between the marker and one vehicle differs from a distance between the marker and the other vehicle. Because of the difference in the distance, an image including the marker and taken by the camera mounted on the one vehicle differs from an image including the marker and taken by the camera mounted on the other vehicle even though the marker is the same. To detect a calibration point, an image processing is performed on a capture region obtained by cutting a part of the image in order to reduce a calculation load for the image processing. When the distance between the vehicle and the marker is shorter, the marker appears larger on the image, accordingly, an area of the capture region is larger. On the other hand, when the distance between the vehicle and the camera is longer, the marker appears smaller on the image, accordingly, the area of the capture region is smaller.
In addition to the difference in a horizontal distance between the vehicle and the marker, a difference in an installation height of the camera varies depending on the vehicle models, which leads to the difference in how the marker appears on the image. This is because a vehicle height varies depending on the vehicle models, and thus the installation height of the camera is not uniformly specified. When the vehicle is parked on a basis of a rear end thereof instead of the front end, an installation position or a direction of the optical axis of the camera varies depending on the vehicle models because the vehicle height or an angle of a rear panel of the vehicle differ depending on the vehicle models.
Since a position of the marker on the image varies depending on the vehicle model as described above, the capture region needs to be set in a calibration device for each vehicle model. Also, size, a shape or the position of the marker on the image varies depending on the position of the marker on the image, and thus the number of pixels contained in the capture region varies. In other words, the area or the position of the capture region defined on the image is affected and changed by three-dimensional factors related to the changes in the height of the camera, in an angle of the optical axis of the camera and in the distance between the marker and the vehicle. Consequently, the image processing (an image processing program) for detecting the calibration point requires an appropriate tuning for each vehicle model, thereby increasing complexity of software. That is, the use of the software that requires the appropriate tuning for each vehicle model leads to complicated maintenance procedures or quality management of the software. Further, a detection performance of the calibration point is affected by the changes in the area of the capture region, making it difficult to obtain stable detection accuracy. However, when the position of the marker is changed for each vehicle model in order to set the same capture region, settings made in the vehicle assembly plant need to be changed for each vehicle model, thereby considerably decreasing productivity.
A need thus exists for a calibration device for an on-vehicle camera, which is not susceptible to the drawback mentioned above.